Homeostasis and Transport

HOMEOSTASIS AND TRANSPORT

Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells

Some substances can cross the cell membrane without any input of energy by the cell

The movement of such substances across the membrane is known as passive transport

To stay alive, a cell must exchange materials such as food, water, & wastes with its environment

These materials must cross the cell or plasma membrane

PlasmaMembrane

Phospholipid bilayer (2 parts)

(allows some molecules to move across cell membrane)

Small molecules like water, oxygen, & carbon dioxide

can move in and out freely

Large molecules like proteins & carbohydrates cannot

move easily across the plasma membrane

The Cell Membrane is semi permeable or

selectively permeableonly allowing certain molecules

to pass through

Diffusion

The net movement of a substance (molecules) down

a concentration gradient from an

When the molecules are even throughout a space -

Passive transport:

Facilitated diffusion:

Diffusion continued…………

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration

Small molecules can pass through the cell membrane by a process called diffusion

Diffusion across a membrane is a type of passive transport because it does not require energy

Diffusion continues until the concentration of the molecules is the same on both sides of a membrane

Diffusion is driven by the kinetic energy of the molecules

Kinetic energy keeps molecules in constant motion causing the molecules to move randomly away from each other in a liquid or a gas

The rate of diffusion depends on temperature, size of the molecules, & type of molecules diffusing

Molecules diffuse faster at higher temperatures than at lower temperatures

Smaller molecules diffuse faster than larger molecules

Osmosis

3 kinds of Osmosis in cells:

Solutions………

Hypertonic Solution

1. Solute concentration outside the cell is higher (less water)
2. Water diffuses out of the cell until equilibrium is reached
3. Cells will shrink & die if too much water is lost
4. Plant cells become flaccid (wilt); called plasmolysis

Hypotonic Solution

Solute concentration is less inside the cell (more water)

Water moves into the cell until equilibrium is reached

Animal cells swell & burst (lysis) if they take in too much water

~~ Cytolysis is the bursting of cells

Plant cells become turgid due to water pressing outward against cell wall

Turgor pressure in plant cells helps them keep their shape

Isotonic Solutions
Concentration of solutes same inside & outside the cell

Water moves into & out of cell at an equal rate so there is no net movement of water

Facilitated Diffusion

• Occurs down a concentration gradient
• Involves carrier proteins embedded in a cell’s membrane to help move across certain solutes such as glucose

Active Transport

Bulk Transport

• Large molecules, food, or fluid droplets are packaged in membrane-bound sacs called ______.

• ______ moves large particles into a cell

There are two forms of endocytosis:

a.)

b.)

• ______is used to remove large products from the cell such as wastes, mucus, & cell products

Exocytosis is the movement of a substance out of a cell via vesicles.Exocytosis is the reverse of endocytosis